The present invention relates to a power control and link adaptation method for compensating for a long round trip delay time and slow channel fading in a long term evolution (LTE)-based mobile communication system having a long round trip delay time and a peak to average power ratio (PAPR) sensitive characteristic, similar to a satellite mobile system.
In the future, a mobile communication network is expected to evolve towards a combination of or cooperation between a terrestrial network and a satellite network. In an integrated satellite and terrestrial system, a commonality between a satellite and a terrestrial wireless interface is an important factor to be considered in terms of a cost for a terminal. In particular, in view of an LTE-based terrestrial mobile system being considered a next generation international mobile telecommunication (IMT)-advanced system, there is a great demand for research on an LTE-based satellite wireless interface which has an even larger cell radius and longer round trip delay time, when compared to a terrestrial network, and is in consideration of a power limited satellite network environment.
A power control is essential to transmit required power for supporting a specific data rate in a mobile communication system, for example, a current cellular communication system. An overly great amount of power may cause unnecessary interference. An extremely small amount of power may increase data transmission errors and a demand for retransmission, and consequently cause a longer transmission delay and a lower processing rate. In a case of a wideband code division multiple access (WCDMA)-based mobile communication system, a power control may be an important factor to be considered in resolving a near-far problem, in particular, in an uplink. In the case of the WCDMA-based mobile communication system, an uplink transmission may not have orthogonality. Accordingly, unless the power control is performed precisely, a strong signal level of user equipment (UE) in a cell center region may cause interference in a weak signal of UE in a cell boundary region. In addition, a power control may be considered for a WCDMA downlink in order to maintain a fixed data rate and delay time, in a similar manner. However, in a case of high speed data transmission, for example, high speed packet access (HSPA), a link adaptation by which a terminal transmits full power and adjusts a data rate based on a channel state may be used in general, rather than the power control.
The link adaptation may refer to a technology for adjusting and adapting a transmission parameter based on a change in a wireless channel state, and may be employed to increase a transfer rate, system capacity, and frequency efficiency. Representative technologies for implementing a link adaptation may include a power control, a hybrid automatic repeat request (H-ARQ), and an adaptive modulation and coding (AMC).
A power control directly related to the present invention is employed solely for a link adaptation in a second generation CDMA scheme, and corresponds to a technology for assuring an overall uniform quality by assigning a much higher transmit power to a terminal distant from a base station than a terminal close to the base station. A power control in the CDMA scheme may be associated with an issue of maximizing a capacity of the base station. In particular, when a transmit power of a wireless terminal positioned at a cell boundary is increased in order for the base station to allow a higher transmit power to a distance wireless terminal than a close wireless terminal for quality assurance, interference in an adjacent cell may be caused. Minimal control of the transmit power minimally may be referred to as the power control.
The power control may be divided into an uplink power control and a downlink power control, and the uplink power control may be classified into an open-loop power control and a closed-loop power control.
The uplink power control may refer to a control of a transmit power of a mobile station, and a technology for enabling all mobile stations to maintain fine communication quality and maximize capacities. A transmit power of a mobile terminal may be controlled in order to produce a minimum signal-to-noise ratio, by equalizing power received by a base station from each mobile terminal.
The downlink power control may refer to a control of a transmit power of a base station, for example, a reduction of interference between mobile communication cells, and correspond to a scheme of outputting a relatively large amount of transmit power to a mobile station distant from the base station and outputting a relatively small amount of transmit power to a mobile station close to the base station.
The open-loop power control may refer to a technology for controlling a transmit power mainly by means of a mobile terminal without forming a loop between the mobile terminal and a base station, and correspond to a scheme of performing a power control on the assumption that a forward path loss is equal to a backward path loss.
The closed-loop power control may refer to a technology for correcting power control errors occurring in the open-loop power control. In the closed-loop power control, a base station may determine a power control bit by comparing a received Eb/No and a requested Eb/No at predetermined intervals, and provide instructions to a mobile station.
In a case of an uplink of the LTE-based terrestrial mobile system, a single carrier frequency division multiple access (SCFDMA) having orthogonality may be used and thus, a near-far problem may not occur, in contrast to the WCDMA-based mobile communication system. However, a high level of interference from an adjacent cell may still restrict an uplink coverage unless a power control is performed on UE of the adjacent cell. In particular, when a level of the interference from the adjacent cell increases, an interference of thermal (IoT) limiting a coverage of a target cell may increase. Accordingly, although the uplink power control may maintain the orthogonality in a cell, paying consideration to the interference from the adjacent cell may be advantageous. However, since the interference may be absent in the cell, an outstanding power control corresponding to the power control performed by the WCDMA-based mobile communication system may be unnecessary. In a case of a downlink of the LTE-based terrestrial mobile system, a link adaptation technology for supporting a maximum data rate based on a channel state at a maximum power, through a link adaptation corresponding to a channel state, rather than a power control, may be employed for high speed transmission.
As described above, the LTE-based terrestrial mobile system may be subject to the no near-far problem since little interference is present in a cell, when compared to the WCDMA-based terrestrial mobile system. In addition, since the link adaptation technology for high speed transmission is regarded to be a main technology, only a power control for resolving interference between cells may be an important consideration.
However, in a case of a mobile system having a long round trip delay time, similar to a satellite mobile system, when a signal transmission through a precise power control at a considered modulation and coding scheme (MCS) level is not performed, a service delay time may increase rapidly due to a relatively long round trip delay time for retransmission. Accordingly, reduction of a number of times the retransmission is performed through the precise power control may be important. In addition, by resolving a decreasing accuracy of a power control instruction for a closed-loop power control, resulting from the long round trip delay time when the power control is performed, the closed-loop power control may be performed precisely. In general, a compensation for the long round trip delay time considered for the uplink power control may not be reflected in a downlink power control not performing the power control. Accordingly, in order to compensate for a long round trip delay time, determination of an MCS level corresponding to a channel state based on the long round trip delay time may be necessary.